The present invention relates generally to resolver systems and deals more specifically with the generation of two-phase (sine/cosine) signals for use with a resolver-to-digital converter (RDC).
It is well known to use a resolver to provide information representative of the angular position of a shaft, such as a motor shaft, or the linear position of a "slider" in a linear system of motion. Known resolvers include the optical type which use multi-track coded aperture disks in which each finite angular position of a shaft is uniquely represented by a binary coded decimal (BCD) value. Since each position in 360.degree. of rotation is uniquely coded, there can be no ambiguity within a full revolution, however, the number of positions that can be coded are limited to the number of apertures which can be placed on the disk.
Other known resolvers are of the electromagnetic type which generate a sine and cosine signal as a function of shaft position angle and may produce a single cycle sine/cosine signal for each shaft revolution or multi-cycle sine/cosine signals for each shaft revolution. Often, a differential multi-cycle resolver is used to prevent ambiguity in multi-cycle resolvers wherein there are as many angular shaft positions as there are electrical cycles in a shaft revolution. A differential multi-cycle resolver is in fact two resolvers wherein the electrical cycles per revolution of each resolver have a difference of one. The combined information provided by the two resolvers provide a non-ambiguous angular shaft position for each of the shaft positions in one revolution of the shaft. It is generally not desirable to use a differential multi-cycle resolver to determine angular shaft position due to its complexity, expense, etc.
There exist a number of other methods and apparatus to generate two-phase (sine/cosine) signals, one of which includes the classical sine/cosine resolver. In the classical sine/cosine resolver, a stator, generally comprised of a set of magnetically permeable poles, is coupled through a radial air gap to a rotor which has a pair of magnetically permeable poles disposed 180 mechanical degrees from one another. The stator poles are disposed 120 mechanical degrees from one another in the case of a three phase output such as typically produced by a synchro. In the case of a resolver, the stator poles are disposed from one another at an angle equal to 90 mechanical degrees to produce a two phase output.
The operation of a classical sine/cosine resolver is generally well known and typically involves the excitation of the rotor by an AC carrier signal which is in the order of several kilohertz. As the rotor rotates through an angle of 360 mechanical degrees, the carrier signal which excites the rotor induces in the stator poles a signal which is modulated by the appropriate trigonometric function of the angle of the shaft position. Accordingly, the stator signals in a two phase resolver are sine and cosine modulations of the AC carrier signal. In the case of a three phase resolver (synchro), the stator signals are sine modulations of the AC carrier signal wherein the sine functions are electrically displaced by 120 electrical degrees from one another.
The stator signals may be decoded by a resolver-to-digital converter (RDC); however, the RDC expects to receive a sine/cosine signal and the three phase stator output signals must be converted to a two phase signal. Such conversion is generally well known and may be made typically by a Scott-T transformation utilizing electromechanical or electronic devices. The resultant two phase (sine/cosine) signal is then converted to an electrical angle and is generally represented in a digitally encoded format.
The resolution and accordingly, the accuracy of the classical sine/cosine resolver can be improved by increasing the number of sets of stator poles to increase the number of electrical cycles per shaft revolution. Since there is a physical limitation to the number of sets of stator poles that may be located about 360.degree., the number of electrical cycles may be increased by providing a number of salients or teeth on each stator pole and likewise providing a number of salients or teeth spaced equidistant from one another about the circumference of the rotor for coaction with the teeth on the stator poles. The number of electrical cycles in a 360 mechanical degree rotation of the rotor will accordingly correspond to the number of rotor teeth. Since the rotor is excited by the AC carrier signal, carrier signals are induced in the stator poles and are modulated by the corresponding trignometric function of the shaft position angle.
Another method for determining the angular position of a rotating shaft is to observe the air gap permeance between the rotor and stator salients as a function of the shaft angle to which a rotor is coupled. The air gap permeance varies as the cosine of the electrical angle as the rotor moves from one aligned position of another aligned position. Since inductance is directly proportional to permeance, the permeance may be inferred by observing the value of inductance. One known method of inferring inductance is to measure the voltage drop across a small sensing resistor placed in series with the winding about a stator pole. The dominant impedance in the circuit becomes the inductive reactance if the series sensing resistor and the winding resistance are kept low with respect to the minimum inductive reactance. It will be seen that the current in the circuit varies inversely with the inductance and a voltage sensed across the series sensing resistor will be inversely proportional to the inductance and accordingly, to the permeance. Since the current varies inversely as the inductance, relatively complex measuring devices are required to translate the measured current into corresponding sine and cosine signals and will also generally require some type of a "look-up" table to implement the translation.
A further method and apparatus generally known and used to achieve increased resolution includes the utilization of a single cycle resolver with a step up gear mechanism so that one revolution of an input shaft results in many revolutions of the single cycle resolver employed. There are a number of problems generally associated with such a stepped-up single cycle resolver among which are gearing backlash, tooth-to-tooth errors, gear run-out, and a number of errors associated with fabrication of the gears, associated shafts, bearings and mounting methods.
Another known method and apparatus for obtaining increased resolution and accuracy and which overcomes a number of problems associated with the above-mentioned methods and apparatus is disclosed in a copending application assigned to the same assignee of the present invention and entitled RELUCTANCE SYNCHRO/RESOLVER, Ser. No. 043,081 filed Apr. 27, 1987, wherein the cyclic variation and permeance is related to the inductance of the respective phases. The reluctance synchro/resolver disclosed in the above-referenced patent generally overcomes the problems associated with sensing variations in permeance which is inversely proportional to the current flowing through the winding. In the above-referenced patent application, the signals derived from each of the various phases are appropriately added and subtracted by interconnection of the stator windings to provide a sine and cosine signal as a function of the electrical angle of the rotor. The sine and cosine signals are in turn provided to the input of the RDC for decoding.
The known methods and apparatus generally assume harmonic free sine and cosine signals which have exactly equal peak amplitudes and are exactly 90 electrical degrees apart from one another. In reality, the signals are not harmonic free and often times are not 90 electrical degrees apart from one another. Accordingly, there are errors between the actual shaft angle position and the determined shaft angle position.
The difference between the actual and the measured or sensed shaft angle positions may be attributable to one or more of the following:
RDC input signals are not in quadrature;
Harmonics are present in the input signals;
Amplitude inbalance between the two (sine/cosine) RDC signals, and
Reference signal phase shift and quadrature signal presence from speed effects.
It is the general object of the present invention therefore to provide a method and apparatus which generally overcome the above-identified sources of error generally associated with the provision of two-phase (sine/cosine) resolver signals for use in a resolver-to-digital converter.